The outside surface of postal articles is made either of plastics material or of paper, depending on whether the wrapping or the envelope is made of plastics material or of paper, for example such articles may be in the form of letters or of packets.
Bar codes can be printed directly on paper-covered articles, but that is not possible on plastics-covered articles which must initially have paper labels applied to them. Printing ink does not dry fast enough on plastics materials, in particular given the high rates of throughput in postal sorting installations. In order to minimize label consumption, a plastics material detector is generally provided so that labels are stuck only on plastics articles.
Various techniques are already known for detecting plastics articles. One known technique consists in illuminating the article and in measuring the light flux it reflects. The index of reflection of plastics material is greater than that of paper so detection is simple to implement. Nevertheless, that technique can be ineffective when plastics articles have an outside surface that is matt or colored, i.e. when they have an index of reflection that is close to that of paper. In addition, the large amount of dust that is to be found in the environment of a sorting installation interferes with light flux measurements. Another known technique is based on a pneumatic principle which consists in deforming the article under inspection by applying suction and in measuring the pressure variation associated with such deformation. Since the outside surface of a plastics-covered article is generally less rigid than the surface of a paper-covered article, detection is simple to implement. Nevertheless, the effectiveness of that technique is very sensitive to dust and requires the pressure sensor to be cleaned frequently in order to maintain satisfactory measurement accuracy.
In general, the methods presently in use for detecting plastics material present poor reliability rates, thereby wastefully increasing label consumption and thus the consumables cost of a working sorting installation.